Displacement measuring interferometers monitor changes in the position of a measurement object relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths are changing relative to one another, e.g., by translating a stage that includes the measurement object, the measured beat frequency includes a Doppler shift equal to 2vnp/λ, where v is the relative speed of the measurement and reference objects, λ is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel, e.g., air or vacuum, and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2π phase change substantially equal to a distance change L of λ/(np), where L is a round-trip distance change, e.g., the change in distance to and from a stage that includes the measurement object.
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as what are known as “cyclic errors.” The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal and have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2πpnL)/λ and the second harmonic cyclic error in phase has a sinusoidal dependence on 2(2πpnL)/λ. Higher harmonic cyclic errors and sub-harmonic cyclic errors can also be present.
Cyclic errors may arise in interferometers from one or more of various sources. For example, in some interferometers, cyclic errors may arise from imperfections in a polarizing beam splitter cube used to separate the components of an incident optical beam into a measurement beam and a reference beam. Nominally, the optical beam may include two beam components having different frequencies, the components further being orthogonally polarized relative to one another. However, the polarization orientations of the beam components may not be perfectly aligned with the s and p polarization directions of the beam splitter cube, and may in some cases be rotated with respect to the s and p directions. Alternatively, or in addition, either or both of the polarization directions of the optical beam components may change on passing through the optical elements of the system, such as the beam splitter cube for example, due to birefringence in the optical elements. Furthermore, misalignment of optical components in an interferometer or interferometry system may occur over time. Any of these sources of error may lead to leakage of a portion of one beam component into a measurement or reference beam that nominally corresponds only to the other beam component. In some cases, portions of each beam component may be present in both the measurement and reference beams. Leakage of beam components into the measurement and/or reference beams may produce cyclic errors in measured interferometry data.
In dispersion measuring applications, optical path length measurements are made at multiple wavelengths, e.g., 532 nm and 1064 nm, and are used to measure the dispersion of a gas in the measurement path of a displacement measuring interferometer. The dispersion measurement can be used in converting an optical path length measured by a displacement measuring interferometer into a physical length. Such a conversion can be important since changes in the measured optical path length can be caused by gas turbulence and/or by a change in the average density of the gas in the measurement arm even though the physical distance to the measurement object is unchanged.
The interferometers described above are often crucial components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beam with a corresponding reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.
In many lithography systems and other applications, the measurement object includes one or more plane mirrors to reflect the measurement beam from the measurement object. Small changes in the angular orientation of the measurement object, e.g., pitch and yaw of a stage, can alter the direction of each measurement beam reflected from the plane mirrors. If left uncompensated, the altered measurement beams reduce the overlap of the exit measurement and reference beams in each corresponding interferometer. Furthermore, these exit measurement and reference beams will not be propagating parallel to one another nor will their wave fronts be aligned when forming the mixed beam. As a result, the interference between the exit measurement and reference beams will vary across the transverse profile of the mixed beam, thereby corrupting the interference information encoded in the optical intensity measured by the detector.
To address this problem, many conventional interferometers include a retroreflector that redirects the measurement beam back to the plane mirror so that the measurement beam “double passes” the path between the interferometer and the measurement object. The presence of the retroreflector ensures that the direction of the exit measurement is insensitive to changes in the angular orientation of the measurement object. When implemented in a plane mirror interferometer, the configuration results in what is commonly referred to as a high-stability plane mirror interferometer (HSPMI). However, even with the retroreflector, the lateral position of the exit measurement beam remains sensitive to changes in the angular orientation of the measurement object. Furthermore, the path of the measurement beam through optical elements within the interferometer also remains sensitive to changes in the angular orientation of the measurement object.
HSPMIs generally include a quarter-wave retarder in each of the measurement and reference legs to rotate linearly polarized light by ninety degrees following a two passes through the retarder (one to, and a second from, the measurement and reference mirrors, respectively.) In one configuration, the quarter-wave retarder is a zero-order retarder made of quartz and is relatively thick (e.g., a couple of millimeters.) In another configuration, the quarter-wave retarder is a stretched polymer sheet, which by itself is relatively thin (e.g., about 100-200 microns). However, the stretched polymer is generally not sufficiently optically flat to be secured to an interferometer surface without a glass cover plate. The glass cover plate causes the resulting retarder assembly to be relatively thick (e.g., a couple of millimeters or more.)